Cancer-associated fibroblast-like cells (CAFs) in the tumor stroma have been shown to exert a profound influence on the initiation and progression of carcinoma, the most common form of human cancer (Bhowmick et al., 2004; Kalluri and Zeisberg, 2006; Pietras and Ostman, 2010; Rasanen and Vaheri, 2010; Shimoda et al., 2010). Pancreatic ductal adenocarcinoma (PDA) in particular is defined by a prominent stromal compartment, and numerous features ascribed to CAFs promote pancreatic cancer progression and hinder therapeutic efficacy (Mahadevan and Von Hoff, 2007). CAFs enhance PDA growth in allograft models in part via paracrine activation of pro-survival pathways in tumor cells, and inhibition of tumor-stromal interactions limits tumor progression (Hwang et al., 2008; Ijichi et al., 2011; Vonlaufen et al., 2008). Further, the dense extracellular matrix (ECM) associated with PDA obstructs intratumoral vasculature, preventing chemotherapeutic delivery (Olive et al., 2009), leading to new ideas to overcome this stromal “roadblock” (Jacobetz et al., 2012; Provenzano et al., 2012). Beyond drug delivery, recent evidence implicates the tumor stroma in innate drug resistance in numerous tumor types (Straussman et al., 2012; Wilson et al., 2012), and treatment paradigms targeting both neoplastic cells and stromal components are emerging for PDA (Heinemann et al., 2012). While these findings suggest that CAFs in the PDA microenvironment represent a potential therapeutic target, the tumor-supporting features of pancreatic stellate cells (PSCs), the predominant fibroblastic cell type in the tumor microenvironment of the pancreas, remain poorly understood.
PSCs are nestin-positive and resident lipid-storing cells of the pancreas, with an important role in normal ECM turnover (Apte et al., 1998; Phillips et al., 2003). In health, PSCs are in a quiescent state, characterized by abundant cytoplasmic lipid droplets rich in vitamin A, and low levels of ECM component production (Apte et al., 2012). During pancreatic injury, PSCs are activated by cytokines, growth factors, oxidative or metabolic stress and transdifferentiate to a myofibroblast-like cell (Masamune and Shimosegawa, 2009). Activated PSCs lose their cytoplasmic lipid droplets, express the fibroblast activation marker α-smooth muscle actin (αSMA), acquire proliferative capacity, and synthesize abundant ECM proteins. Activated PSCs also acquire an expansive secretome which is starkly subdued in the quiescent state (Wehr et al., 2011). Persistent PSC activation under conditions of chronic injury results in pathological matrix secretion leading to fibrosis, creating a physical barrier to therapy. Further, a reciprocal supportive role for activated PSCs and pancreatic cancer cells has become increasingly appreciated: pancreatic cancer cells produce mitogenic and fibrogenic factors which promote PSC activation, such as platelet-derived growth factor (PDGF), transforming growth factor β (TGFβ), and sonic hedgehog (SHH) (Apte and Wilson, 2012; Bailey et al., 2008). Reciprocally, activated PSCs produce PDGF, insulin-like growth factor 1 (IGF1), connective tissue growth factor (CTGF) and other factors which may promote cancer cell proliferation, survival, and migration (Apte and Wilson, 2012; Feig et al., 2012). Tumor-promoting features are largely restricted to the activated PSC state; the activation process may be reversible as suggested by recent work in hepatic stellate cells (Kisseleva et al., 2012). However, the cellular factors and molecular pathways controlling this process remain elusive.